What type of corrosion is this and what would cause it?
a. Surface corrosion from long-term exposure to weather.
b. Crevice corrosion caused by stacking panels while they were wet (before installation on the roof).
c. Galvanic corrosion from coating aluminum-core panels with galvanized steel. Where the galvanic coating wears thin, moisture catalyzes corrosion.
d. Surface corrosion from a manufacturing defect that left the galvanization too thin.